Capacitive fluid level sensing

ABSTRACT

A capacitive fluid level sensing arrangement for use in a medical device is provided. The arrangement includes at least one pair of conductive plates configured to increase and decrease the amount of electric charge stored in relation to the level of fluid within a fluid maintaining device, such as a reservoir. The conductive plates are electrically connected to a medical device and are configured to measure the charge stored between the plates and thus sense the fluid level. The electric circuit may communicate the measurement to an instrument host arrangement for operating a pump configured to remove fluid from the reservoir and move the fluid to a collector when the level exceeds a preset upper level amount. The instrument host arrangement may stop operating the pump when the fluid level is reduced to a preset lower level amount.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to the field of ocular surgery,and more specifically to managing fluid levels within a reservoir usinga capacitive sensor device for measuring the reservoir fluid levelduring ophthalmic procedures such as the removal of a cataract.

2. Description of the Related Art

Phacoemulsification surgery has been successfully employed in thetreatment of certain ocular problems, such as cataract surgery,including removal of a cataract-damaged lens and implanting anartificial intraocular lens. Phacoemulsification surgery typicallyinvolves removal of the cataract-damaged lens and may utilize a smallincision at the edge of the patient's cornea. Through the smallincision, the surgeon then creates an opening in the capsule, i.e.membrane that encapsulates the lens.

Next, the surgeon may insert an ultrasonic probe, incorporated withinthe phacoemulsification handpiece, through the opening in the cornea andcapsule accessing the damaged lens. The handpiece's ultrasonic actuatedtip emulsifies the damaged lens sufficient to be evacuated by thehandpiece. After the damaged natural lens is completely removed, thehandpiece tip is withdrawn from the patient. The surgeon may now implantan intraocular lens into the space made available in the capsule.

While performing phacoemulsification surgical techniques, such as lensremoval, the surgeon may control a pump to pull fluids from the eye andthrough the handpiece tip. The pump is configured with a tank orreservoir positioned to hold the fluid until the tank fills to a certainpoint or level. During emulsification of the damaged lens, the tip ofthe phaco handpiece may collect fluids from the patient's eye andtransfer the fluids for holding or temporarily storing in the reservoir.As the tip further collects fluid and material, the reservoir may fillwith fluid to a point where the ratio of the volume of air with respectto the volume of fluid in the reservoir is outside of a desirableoperating range. Typically, the desired operating range may dictate aminimum volume required for venting and reflux, a maximum volume toprevent the pump from exposure to fluids or from working into anuncompressible volume, and an intermediate or target volume representinga desired air-to-fluid ratio. During an ocular procedure, theair-to-fluid ratio may reach a point where the reservoir requires“rebalancing,” which involves adding fluid to, or removing fluid from,the reservoir for the purpose of maintaining the desired operationalratio.

During the surgery it may become necessary for the surgeon to be able toremove fluid from a reservoir, or tank, into a waste or collection bagfor the purpose of rebalancing the reservoir. One method for rebalancingthe reservoir, when the fluid level exceeds the desirable operatingrange, involves the outflow of fluid and materials from the reservoirinto a collection bag using a pump. When the fluid reaches a certainlevel the pump is turned on and removes or drains the reservoir.Alternatively, if the fluid level in the reservoir falls below a lowlevel threshold, rebalancing may involve the inflow of fluid from thecollection bag or from an infusion bottle into the reservoir. In eitherarrangement, when the reservoir air-to-fluid ratio is returned withindesirable operating values, indicating the reservoir is ‘balanced’ thepump is stopped which in turn stops the flow of fluid and materials.

Maintaining a proper air-to-fluid ratio or balance within the reservoirmay allow the surgeon to perform various aspiration, vacuum venting, andreflux surgical procedures without interruption. When the reservoirlevel reaches an upper lever threshold, thus requiring outflow orremoval of fluid, the instrument host typically turns on a pump to movethe fluid from the reservoir to the collection bag.

In order to remove fluid, current designs typically determine the propertime to activate a peristaltic reservoir pump by sensing the fluid levelin the reservoir. Today's designs typically involve either a floatmechanism, an optical or sound emitter-sensor system, using for exampleinfrared light and ultrasonic frequencies.

Many of today's designs integrate the reservoir with other components,such as pumps, selector valves, and surgical tubing, into a surgicalcassette system. The surgical cassette system is situated between thehandpiece and collection bag and may provide an interface for a vacuumpump and peristaltic pump operations.

For example, Advanced Medical Optics, Inc. (AMO) of Santa Ana, Calif.offers a phacoemulsification medical system that has dual pumpcapability and employs a specific replaceable surgical cassette thatenables dual pump operation and can be changed after a surgicalprocedure. A dual pump surgical cassette exhibiting an efficientreservoir fluid level sensing arrangement that can manage thereservoir's air-to-fluid ratio by controlling pump operation is highlydesirable. Certain designs may include operating the opening and closingof a valve to allow gravity to empty contents from the reservoir intothe collection bag in lieu of operating the pump.

Controlling mechanized fluid outflow and inflow for a surgical cassettereservoir by sensing devices that enable precise determination of thefluid level within the reservoir for operating a pump is often desirablein an operating room situation. While certain sensor devices havepreviously been offered, reliability in air-fluid reservoir balancing inthese cassettes can at times be imperfect, particularly in preciseoperating environments.

Some previous designs include a float mechanism, which can fail bysticking to the side of the reservoir, or the float may “sink” into thereservoir. Optical and sound mechanisms tend to be costly to deploy, andin certain cases are unreliable when the sensing path is subjected tocondensation, droplets, debris, or foam. It would be beneficial to offera surgical cassette that employs minimal components or components thatefficiently control and maintain the fluid level within the cassettereservoir as required in the ocular surgical environment.

SUMMARY OF THE INVENTION

According to one aspect of the present design, there is provided asurgical system comprising a controller configured to receive electricalsignals and effectuate performance of the surgical system, a reservoir,a capacitive sensing device associated with the reservoir and configuredto sense fluid level in the reservoir, and electrical connectionsbetween the controller and the capacitive sensing device. The capacitivesensing device senses changes in fluid level in the reservoir, andconveys sensed changes to the controller via the electrical connections.The controller selectively alters fluid level in the reservoir based onthe sensed changes received from the capacitive sensing device via theelectrical connections.

These and other advantages of the present invention will become apparentto those skilled in the art from the following detailed description ofthe invention and the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is illustrated by way of example, and not by wayof limitation, in the figures of the accompanying drawings in which:

FIG. 1 illustrates an exemplary phacoemulsification/vitrectomy system ina functional block diagram to show the components and interfaces for asafety critical medical instrument system that may be employed inaccordance with an aspect of the present invention;

FIG. 2A illustrates an exemplary surgical system in a functional blockdiagram that shows the vacuum regulated aspiration components andinterfaces that may be employed in accordance with an aspect of thepresent design;

FIG. 2B illustrates an exemplary surgical system in a functional blockdiagram that shows the pressure regulated infusion components andinterfaces that may be employed in accordance with an aspect of thepresent design;

FIG. 3A illustrates a capacitive fluid level sensing system for asurgical cassette reservoir including an electric circuit where a singlepair of planar plates, i.e. two conductors, forms a capacitor;

FIG. 3B illustrates a capacitive fluid level sensing system for asurgical cassette reservoir including an electric circuit where a singlepair of interleaved plates, i.e. two conductors, forms a capacitor;

FIG. 3C illustrates a capacitive fluid level sensing system for asurgical cassette reservoir including an electric circuit where multipleplanar plate pairs, i.e. two conductors, forms multiple capacitors;

FIG. 3D illustrates the horizontal cross-section for an embodiment wherean optical fluid-sensing chamber forms part of the overall evacuationchamber and a single pair of capacitive plates are positioned in closeproximity to the fluid sensing chamber;

FIG. 3E illustrates an exemplary electric circuit configured as a RCcircuit in accordance with an aspect of the present design;

FIG. 3F illustrates an approximate voltage response for the presentdesign electric circuit in accordance with an aspect of the presentdesign;

FIG. 3G illustrates an exemplary electric circuit configured inaccordance with an aspect of the present design;

FIG. 4A is a functional block diagram illustrating a surgical cassettesystem configured for peristaltic pump outflow operation in accordancewith the present design; and

FIG. 4B is a functional block diagram illustrating a surgical cassettesystem configured for peristaltic pump inflow operation in accordancewith the present design.

DETAILED DESCRIPTION OF THE DESIGN

The following description and the drawings illustrate specificembodiments sufficient to enable those skilled in the art to practicethe system and method described. Other embodiments may incorporatestructural, logical, process and other changes. Examples merely typifypossible variations. Individual components and functions are generallyoptional unless explicitly required, and the sequence of operations mayvary. Portions and features of some embodiments may be included in orsubstituted for those of others.

The present design is directed to sensing fluid levels in a reservoir ina system, such as, but not limited to sensing the fluid level within asurgical cassette's integrated air-fluid reservoir and mechanizedcontrolling of the fluid level within the reservoir. The presentarrangement may include a device, such as a pump (peristaltic, venturi,flow or vacuum based pump, etc.), configured to provide outflow/inflowof fluid from the air-fluid reservoir and move the fluid to/from acollector such as a collection bag for purposes of maintaining properbalance of air and fluid in the reservoir. Any pump known in the art maybe used with the present invention, including, but not limited to,peristaltic, venturi (wherein fluid flowing through a narrowing pipeproduces vacuum as a result of the “Venturi effect”), and/or other flowor vacuum based pumps.

The present design may employ a capacitive fluid level-sensor devicewith the air-fluid reservoir for sensing the level of fluid within thecassette's reservoir. The capacitive fluid level-sensor device may be inany orientation with respect to a fluid maintaining device, such as areservoir, including, but not limited to attached inside and/or outsidethe walls of the fluid maintaining device and/or external to the fluidmaintaining device, but not attached to the device. For example, aphacoemulsification system (“phaco system”) may provide for vacuumregulated aspiration, where a surgeon performing an ocular surgery mayremove a large volume of fluid and material from the patient's eye.Vacuum regulated aspiration may increase the fluid level within thesurgical cassette's reservoir in a relatively short amount of time. Ifthe reservoir receives too much fluid, the level may rise above anacceptable level and may inhibit performance. For example, a rise influid level above certain reservoir fluid connections may cause thephaco system to operate improperly or stop.

During vacuum regulated aspiration the phaco system moves fluid from theeye to a reservoir. In order to remove fluid from the reservoir, thephaco system may operate a pump and/or valve configured to move thefluid from the reservoir and into a collector. The present design'scapacitive fluid level sensing system may include an electric circuitconfigured to measure a change in capacitance, such as a rise incapacitance when the reservoir fluid level increases, and a fall incapacitance when the level decreases from the capacitive fluid levelsensor device. The electric circuit could be configured to measure apreset maximum and/or minimum threshold, or a change rate ofcapacitance. The system produces a control signal to start and stop apump situated between the reservoir and collector.

For example, the system can operate the pump to add or remove fluid fromthe reservoir when the level falls outside of preset thresholds, eitherupper or lower, and stop the pump when the level is restored within thedesired operational range. A surgeon performing an ocular surgicalprocedure may input the desired thresholds via the instrument hostsystem or GUI host prior to surgery. In this way, the present design mayallow the surgeon to focus on the ocular procedure without the need tomonitor and manually adjust the air-to-fluid ratio or balance within thereservoir.

The present design comprises a fluid level sensing arrangement that maybe used with a medical instrument system, such as a phaco system. Thesystem can be provided with a reservoir in a surgical cassette systemtogether with a pump and/or valve to control the flow of fluid to and/orfrom the reservoir. Newer cassettes can support aspiration andirrigation functionality, enabling a surgeon to control the operation ofthe phacoemulsification/vitrectomy system handpiece.

The present design is intended to provide reliable, noninvasive, andefficient fluid level sensing in a medical instrument system for use inefficiently controlling the flow of fluids between the reservoir and thecollector during an ocular procedure.

System Example

While the present design may be used in various environments andapplications, it will be discussed herein with a particular emphasis onan environment where a surgeon or health care practitioner performs. Forexample, one embodiment of the present design is in or with a phacosystem that comprises an independent graphical user interface (GUI) hostmodule, an instrument host module, a GUI device, and a controllermodule, such as a foot switch, to control the phaco system.

FIG. 1 illustrates an exemplary phaco/vitrectomy system 100 in afunctional block diagram to show the components and interfaces for asafety critical medical instrument system that may be employed inaccordance with an aspect of the present invention. A serialcommunication cable 103 connects GUT host 101 module and instrument host102 module for the purposes of controlling surgical instrument host 102by GUI host 101. GUI host 101 and instrument host 102, as well as anyother component of system 100 may be connected wirelessly. Instrumenthost 102 may be considered a computational device in the arrangementshown, but other arrangements are possible. An interface communicationscable 120 is connected to instrument host 102 module for distributinginstrument sensor data 121, and may include distribution of instrumentsettings and parameters information, to other systems, subsystems andmodules within and external to instrument host 102 module. Althoughshown connected to instrument host 102 module, interface communicationscable 120 may be connected or realized on any other subsystem (notshown) that could accommodate such an interface device able todistribute the respective data.

A switch module associated with foot pedal 104 may transmit controlsignals relating internal physical and virtual switch positioninformation as input to the instrument host 102 over serialcommunications cable 105 (although footpedal 104 may be connectedwirelessly). Instrument host 102 may provide a database file system forstoring configuration parameter values, programs, and other data savedin a storage device (not shown), such as upper and lower fluid levelpreset thresholds for the reservoir. In addition, the database filesystem may be realized on GUI host 101 or any other subsystem (notshown) that could accommodate such a file system.

Phaco/vitrectomy system 100 has a handpiece 110 that includes a needleand electrical means, typically a piezoelectric crystal, forultrasonically vibrating the needle. Instrument host 102 supplies poweron line 111 to a phacoemulsification/vitrectomy handpiece 110. Anirrigation fluid source 112 can be fluidly coupled to handpiece 110through line 113. The irrigation fluid and ultrasonic power are appliedby handpiece 110 to a patient's eye, or affected area or region,indicated diagrammatically by block 114. Alternatively, the irrigationsource may be routed to eye 114 through a separate pathway independentof the handpiece. Aspiration is provided from eye 114 by a pump (notshown), via instrument host 102, through lines 115 and 116. Asurgeon/operator may select an amplitude of electrical pulses eitherusing handpiece 110 or via instrument host 102 and GUI host 101.

In combination with phaco system 100, the present system enablesaspiration or irrigation functionality in or with the phaco system andmay comprise components including, but not limited to, a flow selectorvalve, one or more pumps, a reservoir, and a collector, such as acollection bag or a device having similar functionality.

The fluid sensing employed is described with respect to a phaco systemhaving dual pump capability and employing a reservoir, such as the“Signature” system available from Advanced Medical Optics, Inc., ofSanta Ana, Calif. Although the present discussion references operationalfeatures and functionality in context with systems such as the AMO“Signature” System, the present design is not limited to designsinvolving dual pump capability or a replaceable cassette and may applyto virtually any fluid based medical design where accurate fluid levelmeasurement is desirable.

FIG. 2A illustrates an exemplary surgical system in a functional blockdiagram that shows the vacuum regulated aspiration components andinterfaces that may be employed in accordance with an aspect of thepresent design. FIG. 28 illustrates the exemplary surgical systemincluding components and interfaces for pressure regulated infusionfunctions. The present design effectively connects theaspiration-infusion line from the handpiece to the air-fluid reservoir,and the reservoir is also connected to the collector through aperistaltic line. The peristaltic connection between the reservoir andcollector includes a peristaltic pump configured to operate in theclockwise and counterclockwise directions.

Surgical cassette venting system 200 may include a fluid vacuum sensor201, flow selector valve 202, reservoir 204, collector 206, and fluidpathways, such as interconnecting surgical tubing, as shown in FIG. 2.Cassette arrangement 250 may include connections to facilitate easyattachment to and removal from instrument host 102 as well as handpiece110 and vacuum pump arrangement 207. The present design contemplates twopumps, where the surgical cassette arrangement may operate with fluidpathways or other appropriate fluid interconnections interfacing withthe two pumps.

Cassette arrangement 250 is illustrated in FIGS. 2A and 2B to showcomponents that may be enclosed within the cassette. The size and shapeof cassette 250 is not to scale nor accurately sized, and note thatcertain components, notably peristaltic pump 203, interface with thecassette but in actuality form part of the device which the cassetteattaches to. Further, more or fewer components may be included in thecassette than are shown in FIGS. 2A and 2B depending on thecircumstances and implementation of cassette arrangement 250.

Referring to FIG. 2A, handpiece 110 is connected to the input side offluid vacuum sensor 201, typically by fluid pathways such as fluidpathway 220. The output side of fluid vacuum sensor 201 is connected toflow selector valve 202 within cassette arrangement 250 via fluidpathway 221. The present design may configure flow selector valve 202 tointerface between handpiece 110, balanced saline solution (BSS) fluidbottle 112, pump 203, which is shown as a peristaltic pump but may beanother type of pump, and reservoir 204. In this configuration, thesystem may operate flow selector valve 202 to connect handpiece 110 withBSS fluid bottle 112, reservoir 204 and/or with pump 203 based onsignals received from instrument host 102 resulting from the surgeon'sinput to GUI host 101.

Flow selector valve 202 illustrated in FIGS. 2A and 2B provides a singleinput port and may connect port ‘0’ to one of three available portsnumbered ‘1’, ‘2’, and ‘3’. Flow selector valve 202 may also be one ormore pinch valves.

Reservoir 204 may contain air in section 211 and fluid in section 212and fluid may move up or down as indicated by arrow 245. Surgicalcassette system 200 may connect reservoir 204 with collector 206 usingfluid pathways, such as surgical tubing or similar items. In thisarrangement, pump 205 may operate in a clockwise direction in thedirection of arrow 228 to remove fluid from reservoir 204 through fluidpathway 227 and deliver the fluid to collector 206 using fluid pathway229. The present design illustrates a peristaltic pump as pump 205, acomponent within instrument host 102, but other types of pumps may beemployed. This configuration may enable surgical cassette 200 to removeunwanted fluid and/or material from reservoir 204. Fluid may alternatelypass through fluid pathway 223 to pump 203, fluid pathway 225, and intocollector 206 in certain situations.

The fluid pathways or flow segments of surgical cassette system 200 mayinclude the fluid connections, for example flexible tubing, between eachcomponent represented with solid lines in FIGS. 2A and 2B.

Vacuum pump arrangement 207 is typically a component within instrumenthost 102, and may be connected with reservoir 204 via fluid pathway orflow segment 230. In the configuration shown, vacuum pump arrangement207 includes a pump 208, such as a venturi pump and an optional pressureregulator 209 (and valve (not shown)), but other configurations arepossible. In this arrangement, vacuum pump arrangement 207 may operateto remove air from the top of reservoir 204 and deliver the air toatmosphere (not shown). Removal of air from reservoir 204 in this mannermay reduce the pressure within the reservoir, which reduces the pressurein the attached fluid pathway 226, to a level less than the pressurewithin eye 114. A lower reservoir pressure connected through flowselector valve 202 may cause fluid to move from the eye, therebyproviding aspiration. Vacuum pump arrangement 207 and reservoir 204 canbe used to control fluid flow into and out of reservoir 204.

The optional pressure regulator 209 may operate to add air to the top ofreservoir 204 which in turn increases pressure and may force air-fluidboundary 213 to move downward. Adding air into reservoir 204 in thismanner may increase the air pressure within the reservoir, whichincreases the pressure in the attached fluid aspiration line 226 to alevel greater than the pressure within eye 114. A higher reservoirpressure connected through flow selector valve 203 may cause fluid tomove toward eye 114, thereby providing venting or reflux.

FIG. 2B illustrates an optional embodiment illustrating a surgicalcassette system 200 configured for venting and/or reflux operation. TheFIG. 2B design may configure flow selector valve 202 to connecthandpiece 110 with reservoir 204 from port ‘2’ to port ‘0’. Vacuum pumparrangement 207 may operate to provide pressure to reservoir 204 viapressure regulator 209. Applying or increasing pressure using pressureregular 209 of vacuum pump arrangement 207 may move air-fluid boundary213 downward in the direction of arrow 245 causing fluid to flow fromreservoir 204 and/or fluid pathway 226 to eye 114.

Capacitive Fluid Level Sensing

The present design provides an alternative to optical fluid levelsensing techniques, for example infrared sensing, and sound sensingtechniques, such as ultrasonic sensing techniques. The present designincludes a capacitive fluid level sensing technique wherein a capacitivefluid level-sensor device, typically a conductive plate pair forming acapacitor, is employed with a reservoir. The capacitive sensor devicemay connect to an electric circuit configured to measure the capacitanceor electric charge stored by the capacitive sensor device, i.e. betweenthe two conductive plates. The electric circuit may communicate themeasurement as a signal to a phacoemulsification instrument host forpurposes of determining the fluid level based on the measured amount ofcharge stored. In a further embodiment of the present design, thecircuit may communicate the measurement as a signal to a separate orself-contained fluid level control circuit, such as, but not limited tothat shown in FIG. 3G. Based on the level determined by the instrumenthost, a pump may be operated to add or remove fluid from the reservoir.

The capacitance formed between two conductive plates, arranged inaccordance with the present design, may be determined by:

C=ε*A/d   (1)

where C is capacitance, ε is permittivity of the dielectric between thetwo plates, A is the area of the plate, and d is the distance betweenconductive plates. Simply put, Equation (1) shows that capacitance isdirectly proportional to permittivity of the dielectric materialsituated between the plates. The relative permittivity of air to avacuum is 1.00054 or approximately 1.0. The relative permittivity ofwater relative to a vacuum, depending on temperature, etc., isapproximately 80 times greater than air, and salt water is approximately10 times greater. The relative permittivity of the balanced saltsolution (BSS) fluid used for phacoemulsification is significantlygreater than air. The large difference in permittivity between air andBSS may allow the present design's capacitive fluid level sensing systemto measure the fluid level in a reservoir or tank.

Arranging a capacitive fluid level-sensor device with the reservoir maystore an electric charge that changes proportional to the amount offluid stored in the reservoir. The capacitive sensing device may includeparallel or planar plates that may extend from the bottom to the top ofthe reservoir, but may also extend to any location in between.

In the situation where the fluid level rises in the reservoir,increasing in height with respect to the conductive plates, theresulting electric charge stored between the plates increases.Conversely, as the fluid level within the reservoir falls the electriccharge stored decreases. Thus continuously sensing and measuring thecapacitance or electric charge stored at the present design's conductiveplates arranged with the reservoir can efficiently enable determiningthe reservoir fluid level. In summary, the capacitance formed by thepresent design's plate pairs is at a minimum when the reservoir isempty, i.e. full of air, and is at a maximum when the reservoir is full,i.e. full of fluid.

FIGS. 3A-3E illustrate various exemplary embodiments for the presentdesign capacitive fluid sensing system 300. Other configurations of theconductive plates are also envisioned by the present invention,including, but not limited to, different sizes (length, width, etc.),shapes, and/or orientations with respect to each conductive plate and/oreach plate pair and with respect to the reservoir. For example,conductive plates may be positioned on opposite sides of the reservoiror at different areas within the same plane of the reservoir wall.

FIG. 3A shows conductive plate pairs oriented in a single plane. FIG. 3Bshows interleaving of plate pairs that may provide a higher capacitance,and FIG. 3C shows multiple fluid level sensors configured at differentheights. In this arrangement, the present design may provide conductiveplate pairs along the walls of the reservoir, external to the reservoir,or otherwise within the reservoir for measuring fluid level in thereservoir. The present design may sense fluid level at multiple distinctheights within the reservoir by arranging plate pairs at a number ofdiscrete points, such as at a high, middle, and a low position withinthe reservoir. FIG. 3D illustrates the horizontal cross-section for anexisting embodiment in a representative system. In the illustrateddesign of FIG. 3D, an optical fluid-sensing chamber forms part of theoverall evacuation chamber. The optical fluid level-sensing chamber mayextend the full vertical length of the evacuation chamber, not shown inFIG. 3D.

FIG. 3A illustrates a capacitive fluid level sensing system 300 for areservoir 204 that may be internal or external to a device such as asurgical cassette 250. Electric circuit 350 comprises a pair of plates,i.e. two conductors, forming a capacitor. The present design may orientthe two plates in a parallel orientation or planar alignment withrespect to each other as illustrated in FIG. 3A. The plate orientationfor the present design is not limited to parallel or planararrangements, however planar plates may provide advantages when attachedin close proximity to the outside of the cassette as illustrated in FIG.3D.

The plates notably may be part of the instrument into which the cassetteincluding the reservoir is inserted. Plates may therefore be positionedoutside the reservoir, outside the cassette, and on the instrument intowhich the cassette is mounted. An example of this type of mounting oroperation is provided in FIG. 3D. In the case of plates attached to thereservoir, they may be inside or outside the reservoir. Preference maybe outside to prevent a direct connection through conductive fluid tothe electronics. If inside, the plates are electrically isolated fromthe fluid, such as by use of insulation or other isolating methodologyknown in the art.

In one embodiment, capacitive fluid level sensing system 300 includesplate 302 and plate 303 as a pair, and are attached to the outside ofreservoir 204 within cassette 250 separated by a distance d as shown atpoint 304. The present design may electrically connect plate 302 toelectric circuit 350 at point 351 and plate 303 may connect at point352. The connections may be realized using a pogo pin male typeconnector, or equivalent connector, configured to plug into a companionpogo pin female connector provided as part of instrument host 102circuit 350. However, any connection known in the art may be used.Electric circuit 350 may include electrical components, such as passivedevices such as resistors and active devices such as diodes connected toa signal source, such as a square-wave generator to drive the circuitbetween the two plates. Driving the electric circuit in this manner mayallow for measuring the amount of electric charge stored, orcapacitance, by the plate pair capacitor arrangement inside reservoir204.

System 300 may sense and determine the fluid level within reservoir 204in relation to the amount of charge measured between the presentdesign's plate pair, plate 302 and plate 303. The plate pair may beintegrated with reservoir 204 by spraying or coating a conductive painton the inside or outside of reservoir 204. Alternatively, the plates maybe implemented by applying conductive tape, such as copper with anadhesive backing, to the inside or outside of reservoir 204.Furthermore, the plates may be implemented as conductive surfaces inclose proximity to reservoir 204 but built into the surgical instrument.Other connection methods may be employed, including but not limited tosuspending the plates in reservoir 204.

The present design may include an electric circuit 350 to exhibit atypical resistor-capacitor (RC) circuit. The voltage in an RC circuitchanges in response to changes in capacitance and may be determinedusing the following formula:

V _(o) =V _(I)*(1−e ^(−t/RC))   (2)

where V_(o) is the measured voltage between the top of plate 302 and thebottom of plate 303 as illustrated in FIG. 3A. Continuing on withequation (2), V_(I) is the voltage of the square wave applied across theplates, R is the resistance of a resistor configured in series with theplate pair, C is the plate capacitance, and t equals time. Configuringan RC circuit in this manner may allow the present design to measure theRC voltage response of circuit 350 and thus determine the capacitanceusing Equation (2).

Capacitive fluid level sensing system 300 may measure the capacitanceresulting from at least one plate pair using electric circuit 350 andcommunicate a signal, for example the voltage response, indicating anincrease or decrease in capacitance to instrument host 102 as a resultof an increase or decrease in fluid shown at 310. Instrument host 102may control a pump to operate and move fluid from the reservoir to thecollector based on a communicated increase in capacitance, presetmaximum threshold, or capacitance change rate. Similarly, the instrumenthost 102 may control a pump to operate and move fluid from the collectorto the reservoir based on a communicated decrease in capacitance, presetminimum threshold, or capacitance change rate.

FIG. 3B illustrates a capacitive fluid level sensing system 300 for asurgical cassette 250 reservoir 204 including electric circuit 350 wherea single pair of plates, i.e. two conductors, forms a capacitor. Theillustration of FIG. 3B shows two interleaved conductive plates orientedas illustrated in FIG. 3B. The plates in FIG. 3B are termed steppedplates, where stepped plates comprise a main or base plate oriented atone angle with plate steps or protrusions oriented orthogonally to themain or base plate as shown. Operation may be as discussed above,wherein system 300 and the circuitry shown may sense and determine thefluid level within reservoir 204 in relation to the amount of chargemeasured between interleaved plate 305 and plate 306.

FIG. 3C illustrates a further capacitive fluid level sensing system 300where three pairs of conductive plates form three distinct capacitors.The present design may orient the three pairs in a horizontal direction,each pair comprising two parallel plates, each pair at differing heightswithin reservoir 204 as illustrated n FIG. 3C.

In one embodiment, capacitive fluid level sensing system 300 may fix orattach a first set of plates 309, a mid level set of plates 308, and athird lower level positioned set of plates 307 to the inside ofreservoir 204 within cassette 250 separated in a multiple heightconfiguration. Sets of plates 307, 308, and 309 may be electricallyconnected to electric circuit 355 at 356 as shown in FIG. 3C. Drivingthe electric circuit in the manner as previously described for FIG. 3Amay allow for measuring the amount of electric charge stored, orcapacitance, at each plate pair arrangement configured inside reservoir204. System 300 may sense the fluid level within reservoir 204 inrelation to the amount of charge measured between the present designsplates at 309, 308, and 307 as previously described for electric circuit350.

In this configuration, indicating the fluid level has fallen below setof plates 307, 308, or 309, capacitive fluid sensing system 300 maymeasure the capacitance resulting from multiple plate pairs usingelectric circuit 355 and communicate a signal indicating a change (e.g.an increase or decrease in capacitance) at each measurement height toinstrument host 102 as a result of an increase or decrease in fluidshown by arrow 310. The present design may individually detectcapacitance at each plate pair, using individual measuring circuits, toindicate when fluid has reached and covered the plates.

The plate pairs can alternately be connected to form a single capacitorresulting in step changes in capacitance as the fluid covers or uncoverseach plate pair as the fluid level rises or falls. Instrument host 102may control a pump, such as a peristaltic pump, to operate and movefluid from reservoir 204 to the collector based on a communicatedcapacitance at each height. For example, if all three capacitors reporta low capacitance value to instrument host 102, the host may determinethat the fluid level is low and may control the peristaltic pump tooperate and move fluid from the collector or other fluid source toreservoir 204.

The previously described embodiments disclose designs that attachcapacitive plates internally and/or externally to the reservoir. Designsthat involve placement of plates, attached inside or outside of thereservoir, may reduce reliability and potentially become unsafe.Reliability in these designs may be reduced by the need to provideelectrical connections from the plates within the cassette to theelectric circuit. Designs that involve placement of the plates insidethe reservoir may potentially complete a direct connection formedbetween the patient and conductive fluid to the electronics, which canbe undesirable. In order to provide a reliable and safe design, and toreduce total cost, the present design may involve configuring thecapacitive plates with and as part of the instrument host system. Theplates may therefore be integrated in the instrument host system and maybe arranged in close proximity to the holding mechanism, or cavity,where the cassette is located. This eliminates the need for electricalconnections within the reservoir.

FIG. 3D illustrates the horizontal cross-section for one such integratedsystem where a fluid chamber 385, such as a reservoir similar toreservoir 204 shown in FIG. 2A, forms part of the cassette design. Thepresent design may fix plate pair 360, 370 on the outside of fluidchamber 385 as shown in FIG. 3D, where chamber wall 380 is shown, and anelectrical connection may be provided between the plates and theelectric circuit within instrument host 102 (not shown in FIG. 3D).

In the illustration of FIG. 3D, the system may include plates 360, 370on the outside of a small cavity associated with instrument host 102configured to hold fluid chamber 385 flush between the plate pair onceinserted into the cavity by an operator. Flush mounting fluid chamber385 in this manner may minimize the distance between the plates andmaximize the change in capacitance observed while mitigating the needfor unreliable electrical connections by using permanent connectionsbetween the plates and electric circuit configured with instrument host102. In this configuration, as fluid rises in fluid chamber 385, orreservoir, the capacitance formed between plates 360, 370 increases, andconversely, as fluid falls in fluid chamber 385, the capacitancedecreases between plates 360, 370.

FIG. 3E illustrates an exemplary electric circuit 350 configured as a RCcircuit in accordance with an aspect of the present design. The presentdesign may arrange a resistor 390 and a signal source 391 with twoconductive plates 392. The voltage response for electric circuit 350 ismeasured between ‘A’ at point 395 and point ‘B’ at point 394.

An approximate voltage response for the capacitive sensor is illustratedin FIG. 3F. The response is plotted as voltage (on axis 396) versus time(on axis 397). The response curve for plates submerged by fluid is shownas response 398 and the voltage response for plates in air is shown asresponse 399. Measurements for a pair of conductive plates in air versussubmerged in BSS yield approximately a 2 to 1 change in the observedcapacitance.

Electrical circuits may be configured to measure capacitance using anoscillator circuit arranged to vary output frequency in relation tochanges in input capacitance at the plate pair. FIG. 3G illustrates anexemplary fluid level sensing system that may involve capacitive platesto realize variable capacitor 360 and may connect variable capacitor 360to a capacitance to frequency converter 361. Converter 361 may varyfrequency output signal 362 (f(C)) in response to the capacitancemeasured at capacitor 360. Fluid level sensing and control circuit 363may receive frequency output signal 362 and based on this frequencyoutput signal may operate pump 205 by turning it on or off using acontrol signal transmitted over line 364. When control circuit 363processes frequency output signal 362 and turns on pump 205, fluid isremoved from reservoir 204 and moved to collector 206 as previouslydescribed.

Additional circuits may include, but are not limited to, varying outputvoltage, current, pulse width, or duty cycle in response to changes ininput capacitance or a constant current charge measuring circuit.

Although the capacitive plate pair represented in FIGS. 3A, 3C, 3D, and3E illustrate or suggest a rectangular shape for the plates, the shapeof the plates are not limited to geometric or rectangular shapes, andmay be realized using customized shapes. The illustrations that formFIGS. 3A-D are generally not drawn to scale and are for illustrativepurposes.

FIGS. 4A and 4B illustrate two modes of operation for the presentdesign. The first mode is illustrated in FIG. 4A, where capacitive fluidsensing system 400 with surgical cassette 250 may employ peristalticpump 205 to move fluid from reservoir 404 to collector 206 as a resultof a high level of fluid in reservoir 404. In this arrangement, platepairs 306, 307, and 308 all may report a high capacitance to electriccircuit 406 via a connection 405 due to fluid covering the three platepairs. Electric circuit 406 may convert the reported capacitance into avoltage response sufficient to indicate to instrument host 102 tooperate peristaltic pump 205 via connection 407 to pump fluid fromreservoir 404 to collector 206.

As instrument host 102 runs pump 205, the amount of fluid decreases asindicated by arrow 425. As the fluid decreases and plate pair 308 isexposed to air in air space 211, the capacitance reported to instrumenthost 102 decreases. As the fluid level drains below plate pair 307, thereported capacitance further decreases. When air-fluid boundary 215 isreduced below plate pair 306, the reported capacitance may fall below acertain threshold indicating reservoir 404 is drained and the instrumenthost may stop pump 205. Operating pump 205 may move fluid from reservoir404 to collector 206 along the path indicated by arrows 410 a, b, and c.General fluid flow to other parts of the design is shown as arrow B 402.

The second mode is illustrated in FIG. 4B where capacitive fluid sensingsystem 400 with surgical cassette 250 may configure peristaltic pump 205for pumping or moving of fluid from collector 206 and/or fluid pathwaysbetween collector 206 and reservoir 404, to reservoir 404 due to a lowlevel of fluid in reservoir 404. In this arrangement, plate pairs 306,307, and 308 all may report a low capacitance to electric circuit 406via a connection 405 when air-fluid boundary 215 is below plate pair306. Electric circuit 406 may convert the reported capacitance into avoltage response sufficient to indicate to instrument host 102 tooperate peristaltic pump 205 via connection 407 in a counter clock-wisedirection 420 b to pump fluid from collector 206 and/or fluid pathwaysbetween collector 206 and reservoir 404, to reservoir 404.

As instrument host 102 runs pump 205, the amount of fluid increases asindicated by arrow 426. As the fluid level increases and rises aboveplate pair 306, the reported capacitance increases. As the fluid levelrises above plate pair 307, the reported capacitance further increases.When air-fluid boundary 215 rises above plate pair 308, the reportedcapacitance may rise above a certain threshold indicating reservoir 404is full. Operating pump 205 may move fluid from collector 206 toreservoir 404 along the path indicated by arrows 420 a, b, and c asillustrated in FIG. 4B. Again, general fluid flow to other parts of thedesign is shown as arrow B 402.

In sum, the present design of a capacitive fluid level sensing systemprovides for automatic draining or filling of fluid within a reservoirduring an ocular procedure by operating a pump, for example a vacuum,venturi, or peristaltic pump, using capacitive sensing. The presentdesign does not require a fluid float mechanism and thus is free ofincorrect measurements due to a stuck or “sunk” float condition.Further, the presence of BSS beads and condensation on the sides of thereservoir tank that make optical level detection difficult generally donot sufficiently alter the measured capacitance because the fluid volumebetween the plates is not significantly changed.

In general, automatic or semi-automatic operation entails sensing achange in capacitance and either drains fluid from the reservoir orpumps fluid into the reservoir. In any circumstance, the surgeon orother personnel are provided with the ability to run the pumps in anyavailable direction, such as for cleaning purposes.

Other pumping states may be provided as discussed herein and may beemployed based on the desires of personnel performing the surgicalprocedure. Other configurations may be provided, including limiting thevoltage response of the capacitive sensing device to be within a desiredrange, and so forth.

The design presented herein and the specific aspects illustrated aremeant not to be limiting, but may include alternate components whilestill incorporating the teachings and benefits of the invention. Whilethe invention has thus been described in connection with specificembodiments thereof, it will be understood that the invention is capableof further modifications. This application is intended to cover anyvariations, uses or adaptations of the invention following, in general,the principles of the invention, and including such departures from thepresent disclosure as come within known and customary practice withinthe art to which the invention pertains.

The foregoing description of specific embodiments reveals the generalnature of the disclosure sufficiently that others can, by applyingcurrent knowledge, readily modify and/or adapt the system and method forvarious applications without departing from the general concept.Therefore, such adaptations and modifications are within the meaning andrange of equivalents of the disclosed embodiments. The phraseology orterminology employed herein is for the purpose of description and not oflimitation.

1. A medical device, comprising: a plurality of conductive platesforming a sensor positioned in association with a fluid maintainingdevice associated with the medical device; and a controller configuredto receive data from the plurality of conductive plates forming thesensor; wherein the plurality of conductive plates are configured tosense fluid changes in the fluid maintaining device and provide data tothe controller, and further wherein the controller is configured tofacilitate fluid evacuation from the fluid maintaining device associatedwith the medical device based on data received from the plurality ofconductive plates.
 2. The medical device of claim 1, wherein theplurality of conductive plates comprises multiple pairs of conductiveplates.
 3. The medical device of claim 1, wherein the fluid maintainingdevice comprises a reservoir housed within a cassette.
 4. The medicaldevice of claim 1, wherein the fluid maintaining device comprises areservoir, and wherein the plurality of conductive plates are positionedexternal to the reservoir located within the medical device.
 5. Themedical device of claim 1, wherein the fluid maintaining devicecomprises a reservoir, and the plurality of conductive plates arepositioned within the reservoir located within the medical device. 6.The medical device of claim 1, wherein the fluid maintaining devicecomprises a reservoir, and the plurality of conductive plates areintegrated with the medical device and positioned in close proximity tothe reservoir configured to be positioned within the medical device. 7.The medical device of claim 1, wherein the plurality of conductiveplates and controller are configured to sense an increased fluid levelin the fluid maintaining device and evacuate fluid from the fluidmaintaining device subsequent to sensing the increased fluid level. 8.The medical device of claim 1, wherein the plurality of conductiveplates is conductive and forms a capacitive sensor wherein capacitanceis proportional to permittivity of dielectric material between theplates, wherein the dielectric material comprises the fluid.
 9. Themedical device of claim 1, wherein the plurality of conductive platesand controller are configured to sense a decreased fluid level in thefluid maintaining device, causing at least one selected from the groupconsisting of: termination of fluid removal; and providing fluid to thefluid maintaining device subsequent to sensing the decreased fluidlevel.
 10. The medical device of claim 1, wherein the plurality ofconductive plates comprises a plurality of interleaved conductiveplates.
 11. The medical device of claim 1, wherein the medical device isan ophthalmic surgical device.
 12. The medical device of claim 11,wherein the ophthalmic surgical device is a phacoemulsification system.13. A fluid level sensing arrangement, comprising: a pair of conductiveplates positioned in association with a fluid maintaining device; and acontroller electrically connected to the pair of conductive plates;wherein the controller is configured to sense electrical changes in thepair of conductive plates based on different volumes of fluid andselectively provide fluid to and evacuate fluid from the fluidmaintaining device based on the controller sensing electrical changes inthe pair of conductive plates.
 14. The fluid level sensing arrangementof claim 13, further comprising an additional pair of conductive plates,wherein the pair of conductive plates and the additional pair ofconductive plates are employed to sense different fluid levels in thefluid maintaining device.
 15. The fluid level sensing arrangement ofclaim 13, wherein the pair of conductive plates is positioned externalto the fluid maintaining device.
 16. The fluid level sensing arrangementof claim 13, wherein the pair of conductive plates and controller areconfigured to sense an increased fluid level in the fluid maintainingdevice and evacuate fluid from the fluid maintaining device subsequentto sensing the increased fluid level.
 17. The fluid level sensingarrangement of claim 13, wherein the pair of conductive plates areconductive and form a capacitive sensor wherein capacitance isproportional to permittivity of dielectric material between the plates,wherein the dielectric material comprises the fluid.
 18. The fluid levelsensing arrangement of claim 13, wherein the pair of conductive platesand controller are configured to sense a decreased fluid level in thefluid maintaining device and cause fluid to be provided to the fluidmaintaining device subsequent to sensing the decreased fluid level. 19.The fluid level sensing arrangement of claim 13, wherein the pair ofconductive plates comprises a pair of interleaved conductive plates. 20.The fluid level sensing arrangement of claim 13, wherein the fluid levelsensing arrangement is part of an ophthalmic surgical device.
 21. Thefluid level sensing arrangement of claim 20, wherein the ophthalmicsurgical device is a phacoemulsification system.
 22. A surgical system,comprising: a controller configured to receive electrical signals andeffectuate controlled functionality of the surgical system; a reservoirconfigured to contain fluid usable in performing a medical procedure; acapacitive sensing device associated with the reservoir and configuredto sense fluid level in the reservoir; and electrical connectionsbetween the controller and the capacitive sensing device; wherein thecapacitive sensing device is configured to sense changes in fluid levelin the reservoir, convey sensed changes to the controller via theelectrical connections, and the controller is configured to selectivelyalter fluid level in the reservoir based on sensed changes received fromthe capacitive sensing device via the electrical connections.
 23. Thesurgical system of claim 22, wherein the capacitive sensing devicecomprises a pair of conductive plates.
 24. The surgical system of claim23, wherein the capacitive sensing device comprises an additional pairof conductive plates, wherein the pair of conductive plates and theadditional pair of conductive plates are employed to sense differentfluid levels in the reservoir.
 25. The surgical system of claim 22,wherein the capacitive sensing device is positioned in association withthe reservoir.
 26. The surgical system of claim 22, wherein thecapacitive sensing device is positioned external to the reservoir. 27.The surgical system of claim 22, wherein the capacitive sensing deviceand controller are configured to sense an increased fluid level in thereservoir and evacuate fluid from the reservoir subsequent to sensingthe increased fluid level.
 28. The surgical system of claim 22, whereinthe capacitive sensing device is configured to sense capacitance and thecapacitance is proportional to permittivity of dielectric materialbetween the two conductive plates, wherein the dielectric materialcomprises the fluid.
 29. The surgical system of claim 22, wherein thecapacitive sensing device and controller are configured to sense adecreased fluid level in the reservoir, causing at least one selectedfrom the group consisting of: termination of fluid removal; andproviding fluid to the reservoir subsequent to sensing the decreasedfluid level.
 30. The surgical system of claim 22, wherein the pair ofconductive plates comprises a pair of interleaved conductive plates. 31.The surgical system of claim 22, wherein the surgical system is usedduring an ocular surgical procedure.
 32. The surgical system of claim31, wherein the ocular surgical procedure is phacoemulsification.
 33. Amethod for performing a surgical procedure, comprising: capacitivelysensing fluid level within a medical device configured to be employed toperform the surgical procedure; and depending on fluid level sensed bysaid capacitively sensing, selectively altering fluid amount within themedical device by selectively evacuating fluid and selectively addingfluid based on the capacitively sensing.
 34. The method of claim 33,wherein the medical device is an ophthalmic surgical device.
 35. Themethod of claim 34, wherein the ophthalmic surgical device is aphacoemulsification system.